You do know that the companies that make the name brand cereals...also make the store brand cereals...
It's about market fit. Price sensitive customers would generally not buy name brand because it's too expensive, and the brands don't won't lower-priced products to "sully" the image of the brand. Hence, they sell white label (aka store brand) products to retail stores that are generally lower quality and thus cheaper.
Supermarkets frequently charge slotting fees to appear on their shelves.
Yes, some do. For new products that they wouldn't otherwise stock on the shelves, because it's in lieu of the anticipated lost revenue from saleable products that would otherwise have gone on the shelves. Note that slotting fees are used alongside consignment arrangements.
Supermarkets often have contracts where they can return stock to suppliers if it is defective or not selling well and do not always own their stock.
Yes, but they've still paid for those products in the first place (in the legal/accounting sense). Refunds come in the form of discounts or credits on future invoices.